DEVELOPMENT OF CARBON CARBON COMPOSITES BY COCARBONIZATION OF PHENOLIC RESIN AND OXIDIZED PAN FIBERS/

In order to develop cost effective carbon/carbon composites for genera l purpose applications, alternative reinforcements need to be tried to replace high modulus carbon fibers. Present studies deal with applica tion of oxidised PAN fibers as reinforcement with char yielding phenol ic resin and co-carbonisation of the composites. Polyacrylonitrile (PA N) fibers have been oxidised at 230 degrees C for different times in t he presence of air. Characterisation of oxidised fibers for surface gr oup analysis exhibit active groups such as carboxylic, etc., on the fi ber surface increasing with time of oxidation. Pyrolysis of the fibers to 1000 degrees C causes anisotropic shrinkage, whereas the cured res ins exhibit, to a greater extent, isotropic shrinkage. The shrinkage o f the composites during carbonization has been found to be controlled by the shrinkage of the fibers and is observed to be anisotropic, high er in cross-section than in length direction. This results in the deve lopment of stresses in the matrix and at fiber/matrix interfaces. As a result, on examination of the composites under an optical microscope with polarised light, carbonized composites exhibit the development of anisotropic matrix structure initiating at the interface. On heat tre atment to 2800 degrees C, the carbon matrix as well as the fibers exhi bit well-developed graphitic structures with a sharp XRD peak at 2 the ta = 26.4 degrees. Optical microscope and SEM examinations of the comp osites, show strong fiber/matrix bonding, although the two constituent s are clearly distinguishable. In no case has the reaction between the resin and the oxidised PAN fibers leading to coalescence of the two o r destruction of the fibers with the matrix during co-carbonization be en observed. The composites at each stage have been evaluated for flex ural strength. Carbonized composites exhibit low strength with catastr ophic failure whereas the strength of the composites is found to incre ase on graphitization and the fracture is also changed to pseudoplasti c. This is attributed to changing fiber/matrix interactions at differe nt temperature of heat treatment of the composites. Copyright (C) 1996 Elsevier Science Ltd